Cellular radiotelephones are widely used for wireless mobile communications of voice and/or data. As used herein, the term "cellular radiotelephone" encompasses a wide variety of portable radiotelephone devices that access a cellular radiotelephone system. Cellular radiotelephones include portable telephones of the handheld or bag phone variety and permanently mounted car cellular telephones. The term "cellular radiotelephone" also includes wireless terminals that provide functions in addition to those of the cellular telephone, such as facsimile, data communications, data processing, word processing applications and other functions. Added function cellular radiotelephones are often referred to as "Personal Communications Systems" (PCS).
When a cellular radiotelephone is powered on, it performs an initialization procedure with the cellular radiotelephone system. In general, the cellular radiotelephone scans a plurality of channels and/or time slots in order to locate an appropriate control channel. Cellular radiotelephones that operate in the U.S. AMPS system may only need to scan a limited number of channels at power-up in order to locate a broadcast control channel. Broadcast control channels generally are confined to a small portion of the available spectrum about 1 MHz wide in order to reduce scan time. Moreover, since in AMPS the broadcast control channel transmissions generally are continuous transmissions, the receiver could alight on a scanned channel at any time and make a measurement. In analog cellular telephones it is known to scan channels in sequential order to minimize the frequency changing time from one channel to the next.
A current trend is to utilize digital transmission for speech and/or data traffic. A number of digital cellular standards are in use that are based on Time Division Multiple Access (TDMA). TDMA systems include the IS136 (D-AMPS) system and the GSM system, also known as DCS1800 when used in the 1800 MHz band and as PCS1900 when used in the U.S. 1900 MHz PCS bands. Ongoing development of TDMA standards continues to make improvements in service and product utility, such as longer battery life. One feature introduced into the D-AMPS system, for example, is the Digital Control Channel (DCC) which can reduce the standby battery consumption of cellular radiotelephones that are camped on the DCC to await calls. Unlike the AMPS broadcast control channel, the DCC need not be a continuous carrier signal, but occupies only one slot of the 3-slot TDMA frame. The other two slots can contain traffic, but may be empty during periods of low demand. A major concern with such TDMA systems is the continued reduction in the time that is needed to scan the TDMA cellular channels to identify a channel containing at least one TDMA burst transmission.
U.S. Pat. No. 5,197,093, to Knuth et al., describes a cordless telephone which has an improved mechanism for scanning and selecting channels by adapting to the channel usage patterns of the local environment in which it is placed. This results in a prioritized list of channels that have the highest probability of being available, free from interference. By prescanning the channels during the idle time of the cordless telephone, power usage of the handset is minimized and quick acquisition of an available channel becomes possible. However, Knuth is not concerned with identifying a broadcast control channel of a cellular system by finding channels containing the strongest signal, but rather is concerned with finding channels which contain the minimum of interference, i.e. the minimum signal strength.
U.S. Pat. No. 5,511,235 to Duong et al., describes a receiver which has a channel scan mode of operation and a communication mode of operation. In the channel scan mode of operation, the passband of a filter is narrowed relative to the passband of the filter in the communication mode.
U.S. Pat. No. 5,524,280 to Douthitt et al., describes a method of scanning channels that includes fast scanning a predetermined list of data channels to identify a fast scanned channel; intermediate scanning, when the fast scanned channel is not identified, the predetermined list of channels to identify an intermediate scanned channel where a channel from the predetermined list of channels is evaluated for a first time period; and slow scanning, when the intermediate scanned channel is not identified, the predetermined list of channels is scanned to identify a slow scanned channel where a channel from the predetermined list of channels is evaluated for a second time period.
U.S. Pat. No. 5,574,995 to Masaki describes a controller that shifts the frequency of the local oscillator with a frequency shift circuit so that as many channels to be scanned as possible will be included within specified bandwidths, detects a desired received signal by scanning each of the specified bandwidths with the shifted frequency, scans each channel within the bandwidth where the desired received signal was detected, and changes the frequency of the local oscillator or shifts the frequency of the local oscillator with the frequency shift circuit so that the desired channel frequency identified above will be at the center of said bandwidth. However, Masaki is apparently not concerned with being able to decode a detected signal in the presence of an adjacent channel signal, as is the case in cellular systems. Rather, Masaki appears to be concerned with detecting a signal in a sparsely populated part of the spectrum where adjacent channel discrimination may not be needed, and of identifying the frequency channelisation raster on which a detected signal is allocated. Masaki adapts the receiver to the identified channelisation raster so that a detected signal is centrally placed in the receiver bandwidth, while avoiding the need for a plurality of receiver bandwidths adapted to different channelisation rasters.
Multiple-mode cellular radiotelephones such as dual-mode cellular radiotelephones are also widely used in cellular radiotelephone communications. For example, dual-mode cellular radiotelephones may operate both in narrowband FM mode and in wideband CDMA mode. Alternatively, dual-mode cellular radiotelephones may also operate in a narrowband cellular standard mode such as AMPS or D-AMPS (IS136) as well as a wider bandwidth standard such as GSM (known in the U.S. as PCS 1900) or IS95. The narrowband standard may use a receive channel spacing of 30 kHz while the GSM/PCS 1900 standard may use 200 kHz channel spacing.
Dual-mode cellular radiotelephones generally are adapted in bandwidth to operate in different channelisation rasters, for example a 200 kHz raster for GSM operation and a 30 kHz raster for D-AMPS operation. Moreover, receiver bandwidths of dual-mode cellular radiotelephones are generally narrower than the channel spacing for which they are intended, for example 20% less than the channel spacing. This contrasts with Masaki, in which the receiver bandwidth is wider than the expected channel spacing.
Accordingly, there continues to be a need for cellular radiotelephones and methods that can efficiently acquire a channel. There is a particular need for improved channel acquisition in multiple-mode cellular radiotelephones.